The present invention relates generally to the determination of various downhole parameters of a wellbore penetrated by a subterranean formation. More specifically, the present invention relates to techniques for determining downhole pressure during wellbore operations.
In a typical wellbore operation, a downhole drilling tool drills a borehole, or wellbore, into a rock or earth formation. During the drilling process, it is often desirable to determine various downhole parameters in order to conduct the drilling process and/or learn about the formation of interest. The downhole drilling tool may be provided with mechanisms for measuring and/or monitoring such downhole parameters. To further investigate the wellbore and the downhole parameters of interest, the drilling tool is removed and a wireline tool is lowered into the wellbore to take measurements and/or to take samples. Such techniques for determining downhole parameters are sometimes referred to as “formation evaluation.”
Present day oil well operation and production involves continuous monitoring of various subsurface formation parameters. One aspect of standard formation evaluation is concerned with the parameters of downhole pressures and the permeability of the reservoir rock formation. Monitoring of parameters, such as pore pressure and permeability, indicate changes to downhole pressures over a period of time, and is essential to predict the production capacity and lifetime of a subsurface formation, and to allow safer and more efficient drilling conditions. Such downhole pressures may include annular pressure (PA or wellbore pressure pw), pressure of the fluid in the surrounding formation (Pp pore pressure), as well as other pressures.
During drilling of oil and gas wells using traditional downhole tools, it is common for the drill string to become stuck against the formation. A common type of sticking, known as differential sticking, occurs when a seal is formed between a portion of the downhole tool and the mudcake lining the formation. The pressure of the wellbore relative to the formation pressure assists in maintaining the seal between the mud cake and the downhole tool, typically when the tool is stationary. The hydrostatic pressure acting on the downhole tool increases the friction and makes movement of the drill pipe difficult or impossible. Monitoring downhole pressure conditions enables detection of the downhole pressure conditions likely to result in differential sticking.
Techniques have been developed to obtain downhole pressure measurements through wireline logging via a wireline, or “formation tester,” tool. This type of measurement requires a supplemental “trip” downhole with another tool, such as a formation tester tool, to take measurements. Typically, the drill string is removed from the wellbore and a formation tester is run into the wellbore to acquire the formation data. After retrieving the formation tester, the drill string must then be put back into the wellbore for further drilling. Examples of formation testing tools are described in U.S. Pat. Nos.: 3,934,468; 4,860,581; 4,893,505; 4,936,139; and 5,622,223. These patents disclose techniques for acquiring formation data using probes positionable in fluid communication with the formation of interest.
A conventional technique for measuring pressure involves withdrawal of formation fluid and measuring the pressure relaxation with time. In conventional methods, tools with probes engage and press to the wall of the formation creating a seal and establishing fluid communication between the probe and the formation. In order to make an accurate formation pressure measurement, it is necessary to remove the mudcake, at the probe location. The mudcake removal step is necessary because mudcake prevents communication between the probe and formation (there is external mudcake at the interface of the formation but there can be some mudcake that penetrates the formation).
For the mudcake removal operation, the probe is typically provided with a pump used to create the underbalanced pressure that will cause the formation fluid to flow from the formation into the tool through the probe. This process cleans the interface between the tool and the formation and establishes communication between the tool and the formation. In conventional pressure measuring terms, this process is known as the “pretest” phase. When the fluid begins to flow into the probe, the pressure in the probe (the cavity) will drop quickly from the wellbore pressure below the formation pressure. After the pressure relaxation occurs, the pressure in the probe will start to increase. The pressure in the probe rises to a point that is close to the formation pressure.
The pretest operation reduces the pressure around the wellbore. Therefore, it is necessary to wait until the pressure relaxes/equalizes to a level close to the formation pressure. If substantial fluid is withdrawn during the pretest, but not enough time is allowed for the formation pressure to relax, then the pressure measurement will be inaccurate. The time required to wait for pressure relaxation can vary depending on the permeability of the formation and fluid viscosity. This wait can be on the order of several minutes. This delay, with the tool in a stationary position, may further result in disruption of the downhole operation and potentially cause the downhole tool to stick in the wellbore.
Most jobs involving pore-pressure measurement currently performed during drilling operations with commercial wireline tools are time-consuming operations. Among the main causes for this time consumption are the pretest and pressure equalization procedures, even when the formation permeability is high and the pressure behind the mudcake adequately represents the formation pressure. As previously mentioned, the removal of mudcake (external and internal) requires the withdrawal of filtrate and reservoir fluids from the formation, whereas the interpretation of the pretest requires monitoring pressure variations inside the cavity.
Since the cavity in the tool is about 100 cm3 and the storage effect on the pressure inside the tool cannot be ignored, the amount of fluid that has to be withdrawn from the formation is relatively large. Usually, the amount of fluid withdrawn is about 20 cm3. This withdrawal, in turn, leads to a significant pressure perturbation around the wellbore and, therefore, additional waiting time is necessary for subsequent pressure relaxation in the formation.
Other techniques have also been developed to acquire formation data from a subsurface zone of interest while the downhole drilling tool is present within the wellbore, and without having to trip the well to run formation testers downhole to identify these parameters. Examples of techniques involving measurement of various downhole parameters during drilling are set forth in U.K. Patent Application GB 2,333,308 assigned to Baker Hughes Incorporated, U.S. patent application Ser. No. 6,026,915 assigned to Halliburton Energy Services, Inc. and U.S. Pat. No. 6,230,557 assigned to the assignee of the present invention. Like the conventional wireline tools, these measurement devices also typically require removal of the filtercake to establish sufficient communication between the tool and the formation, and require a substantial delay for equalization of pressure before taking a measurement resulting in similar delays and potential sticking.
Still other techniques have been developed to measure pore pressures without requiring a pretest. For example, U.S. Pat. No. 6,164,126 assigned to the assignee of the present invention discloses an apparatus and method for measuring formation parameters using a probe embedded in the formation. However, this device requires delays to equalize the pressure of the device with the pore pressure to obtain an accurate measurement of the formation. Additionally, the device must penetrate the formation to obtain the measurement thereby causing damage to the formation which may impair fluid communication between the device and the formation, and which may cause difficulty in the operation of the drilling tool.
The acceleration of pore-pressure measurements following conventional procedures poses a significant hurdle. What is needed is a more efficient way to equalize the pressure on both sides of the tool-rock interface. The method of equalizing pressure in a timelier manner remains a paramount challenge. Therefore, there remains a need to further develop techniques which permit quick and accurate downhole pressure measurements. The downhole drilling operation, known pressure conditions and the equipment itself may be manipulated to facilitate downhole measurements.
It is desirable that techniques be provided to take pore pressure measurements using reduced volumes of formation fluid to generate quick pressure readings with reduced damage to the formation during testing. It is further desirable that such techniques be capable of providing one or more of the following, among others, adaptability to various wellbore and/or equipment conditions, usability in a wireline or drilling tool, capability of utilizing a reduced volume of formation fluid necessary for testing, capability of reducing the damage to the wellbore during testing, improved accuracy, simplified equipment, real time data, elimination of sticking risks, quick measurement and/or measurements during the drilling process. Such a technique would also preferably be adapted to penetrate the mudcake and take an immediate pressure measurement, measure pore pressure with minimum formation pressure disturbance, measure the pore pressure at the interface between the mudcake and rock during drilling operations, establish effective hydraulic communication between the formation and the tool during the measurement operation, and provide minimum disturbance of pore pressure around the wellbore.